Magnetic resonance systems acquire imaging and/or spectrographic data using strong magnets for providing large static magnetic fields. Gradient coils within the magnet are provided to "focus" the magnetic fields. The large static magnetic fields are used to magnetically align certain nucleii ("spins") of the sample being imaged, or spectroscopically studied. A radio frequency (RF) pulse is used to "tip" the aligned spins so that at least a projection of the tipped spins is in a plane orthogonal to the plant in which the spins are aligned. The tipped spins rotate or precess in the orthogonal plane at the Larmor frequency which is equal to .gamma.B/.pi., where:
B is the strength of the static magnetic field PA1 .gamma. is the gyromagnetic constant/element, and PA1 .pi. is the constant 3.1416.
A decaying signal known as a free induction decay signal (FID) is generated by the rotating spins cutting lines of force in the magnetic field. The decay occurs because when the RF pulse terminates the nutated or tipped spins tend to dephase in the orthogonal plane and also tend to return to the aligned condition. It is the FID signals in one form or another that are used for imaging and/or spectroscopic purposes.
There are many types of magnets which can be used to generate the large static magnetic field; in a preferred embodiment, a superconducting magnet is used. The subject or patient is placed in the bore of the superconducting magnet for exposure to the large static magnetic field.
Radio frequency coils are used for transmitting RF pulses and/or receiving the FID signals. These coils are energized in a transmitting state with the RF pulse at the Larmor frequency.
The RF coils are either body coils wound within the large magnet, or special coils often used in addition to the body coils. Special coils are designed to be juxtaposed to particular problems of the body such as spine, limbs or the head. Surface coils are such special coils designed to be juxtaposed to particular portions of the body. Surface coils are relatively efficient due to the proximity of the probe to the body part from which data is acquired.
Notwithstanding the relatively high efficiency of the special coils including surface coils; the signal-to-noise ratio (SNR) of the acquired data remains critical because of the inherently small amplitudes of the FID signals. The SNR is decreased because, among other things, "pickup" of stray signals (noise) by the coil caused by stray capacitances and/or mutual inductances between the coils in quadrature surface coil arrangements or in surface coil array arrangements.
The SNR is also decreased because of variations in the impedance of the coil due to "loading" by the patient. Different patients have different body impedances; and, therefore, load the RF probes differently. Also, the human body is asymmetrical. Thus, the coil loading is not symmetrical which results in variations in the signals received from the coil at different locations in the body. SNR is also adversely affected by the size of the surface coil; so that when other things are equal, the larger the surface coil the smaller the signal-to-noise ratio.
A surface coil array is described and claimed in copending application Ser. No. 587,447 filed Sep. 25, 1990.
A problem with RF coil arrays used in MR systems in that the MRI system uses quadrature detection; i.e., the detection is applicable to both the dispersion mode and the absorption mode each of which are 90.degree. apart. Another way of describing the orientation of the coils is that the quadrature detection provides real data and imaginary data. An exact balance between each of the coils is essential to prevent reflection, among other things. The equipment described herein enables a relaxation of the requirement for "exact balance", among other things.
Accordingly, it is the object of the present invention to provide circuitry to supplement a surface coil array by preventing reflection. In the prior art receiver circuitry for surface coil arrays it is necessary to spend a good deal of time in balancing the circuitry coupling the RF coil arrays to the image processing equipment to prevent unbalance between the signals received from the different individual coils of the array of coils.